Dark beasts are large, bull-like creatures found in the Mourner Tunnels on the path to the Temple of Light and inside Kuradal's Dungeon. Dark beasts require a Slayer level of 90 to kill. They can attack using magic or melee. If the player is not adjacent to the beast, it only uses magic and does not try to move into melee range. If the player is within melee range, it uses melee most of the time. These monsters are weak to bolts.

While dark beasts can only be accessed by members, their lower-level revenant counterparts are on both member and free to play worlds. If assigned as a Slayer task, Revenant dark beasts may take their place. However, killing their revenant counterparts is not a good idea because the profit between the regular and revenant version is extremely varied, the revenant counterpart respawns somewhat slow and players risk all of their items within the wilderness.

Dark beasts are an often underestimated method of training combat stats. Using good ranged armour, a good crossbow and bolts, and potions (stat boosting and prayer potions), players may kill these beasts swiftly, as their defence is only average. The cost of buying the needed potions may be offset by dark bow drops.

Dark beasts have a higher chance (14%) to drop blue charms than most other creatures, exception being glacors, which have a 35% chance of dropping blues and tormented demons, which have a 40% chance of dropping 3 blue charms. However, glacors and tormented demons have more requirements than dark beasts.

When entering from the headquarters, the player must wear mourner gear and have the new key to enter the tunnels from the mourner headquarters basement. Mourner gear can be obtained during and after the Mourning's Ends Part I quest, but the player must have started the Mourning's Ends Part II quest to obtain the new key and enter the tunnels.

The new key is easily obtained in the mourner headquarters and can be readily discarded once in the tunnels to free up an inventory slot. Mourner gear is not well suited for fighting dark beasts, so players prefer to switch to ranged gear and heal up once in the tunnels. Mourner gear takes up six inventory slots which could otherwise be used more efficiently. The gear is sufficiently time consuming to obtain, however, so many players prefer not to discard it once in the tunnels. This leads players to seek other routes to the dark beasts.

After Mourning's Ends Part II, players can go to the death altar via the Abyss and then reach the tunnels by going through the Temple of Light. This method requires the crystal trinket from the quest to be in inventory to reset the light beam in the temple. This route is somewhat long and dangerous, however, since both the Abyss and the temple have aggressive monsters.

After the Within The Light quest, players can have Eluned help them add a Temple-of-Light teleport to their crystal teleport seeds. This teleport is a very fast and safe way to reach an area near the dark beasts and avoids cluttering inventory with mourner gear, keys, or trinkets.

After Plague's End quest mourner gear is no longer required to enter the tunnels.

The release of Kuradal's Dungeon has made it possible to receive dark beasts as a Slayer task by players who have not completed Mourning's Ends Part II. The beasts are located in the second to last chamber of the dungeon, immediately before the iron and steel dragons. However, as with all monsters in Kuradal's dungeon, they may only be fought if they have been assigned as a task by Kuradal or Morvran.

The Slayer mask or helm, the Mask of Gloom or Helm of Darkness, provides two daily teleports straight to the Dark beast cave (or the Mourner tunnels).

Players with 90 Agility can take the shortcut near the dungeon entrance to get to them quickly.

The entrance to the dungeon is located in the Ancient Cavern. Players may quickly reach this area by using the ferocious ring's teleport to arrive next to Kuradal or by using the fairy ring code B•J•Q which will take players near the steps leading up to the dungeon entrance.